1. Sprinkle the yeast into ˝ cup of the lukewarm milk. Add the sugar and salt and stir until thoroughly dissolved. Place the mixture in a warm, draft-free place, such as an unlighted oven, for 5 to 8 minutes, or until the mixture has begun to bubble and almost doubled in volume.

2. Pour the flour into a large mixing bowl and make a deep well in the center. Drop in the yeast mixture, egg yolks, and the remaining ˝ cup of milk and 8 tablespoons of the butter and, with a large wooden spoon, slowly stir the flour into the liquid ingredients. Beat vigorously until fairly firm dough is formed.

3. Cover the bowl loosely with a kitchen towel and place it in the warm draft-free spot for 45 minutes to an hour, or until the dough has doubled in bulk. Then punch the dough down with your fist, cover loosely, and let it raise another 45 minutes, or until it again doubles in bulk.

4. Meanwhile, prepare the filling. Stirring occasionally, fry the bacon and chopped onions in a large skillet over moderate heat. When the bacon has rendered all its fat and the onions are tender and lightly colored, drain them through a fine sieve. Discard the fat and set the bacon and onions aside.

5. With a pastry brush, lightly coat 2 cookie sheets with a tablespoon of butter each. Cut the dough in half and, on a lightly floured surface, roll each half into a circle about 1/8 inches thick. With a 2˝- to 3-inch cookie cutter, cut out about 18 rounds of dough.

6. Drop 1 teaspoon of the bacon mixture in the center of each and fold in the sides of the circle, making a neat packet. Set the buns, seam-side down, on the cookie sheet, leaving 1 inch between them. Roll and fill the remaining dough similarly. Place the cookie sheets in the warm, draft-free spot for 15 to 20 minutes or until the buns almost double in size.

7. Preheat the oven to 375°F. Bake the buns in the center of the oven for 10 minutes, then, with a pastry brush, coat each bun with the egg yolk-and-cream mixture. Bake another 10 minutes, or until golden brown. Serve the buns hot; if you cool them after baking, reheat for 10 minutes or so in a 325°F oven.